This invention generally relates to electromagnetic shielding and more particularly relates to a shielded housing for accommodating electronic circuits and/or components which are sensitive to electromagnetic interference fields and/or radiate interference fields.
Usually electronic circuits, sensitive to irradiated high-frequency electromagnetic fields, are accommodated in shielded housings. The shield protects the circuitry against the direct incidence of the high-frequency fields. The electric fields, however, may also reach the circuitry indirectly via electric cables, leading into the housing, and cause electrical interference with the circuitry. Therefore, filter devices which are active against high frequencies are provided at the point where the cable enters the housing. These filter devices are usually discrete feed-through filters or feed-through capacitors, respectively.
The above-described arrangement may also be used with circuits, emitting high-frequency electromagnetic fields, so as to protect the environment against the emission of fields.
Such shielded housings usually are made from metal and the feed-through capacitors are soldered, screwed or pressed in. A noise-suppressing plug is described in U.S. Pat. No. 5,167,539. The walls of the plug receptacle are of metal.
For various reasons (such as cost, weight, elasticity) often plastic housings are used. So as to achieve the desired shielding effect these housings are metallized. The electric connection between feed-through capacitor and metallization often cannot be achieved by simple means.
It is thus an object of this invention to develop a housing of the type referred to at the beginning which is characterized by a simple design, low manufacturing expense and by a high filtering effect, in particular in the area of entry of the cables leading into the housing.
The present invention fulfills the objects of the invention in the area of entry or lead-in, filter devices with capacitors are provided which are embodied as integral components of the connecting elements and of the shield.
This invention renders the use of discrete and constructionally expensive feed-through capacitors superfluous. Instead, the structures in the area of the points of penetration which consist of the electrically conductive connections leading into the housing (such as plug pins and punched grids, injection-moulded into the housing, and other inserts), of the insulating layers (dielectrics) and of the shield serve as feed-through capacitors.
The dielectric may be applied to the conductive connection either out of the housing material during the manufacture of the housing (e.g., plastic injection-moulding processes) or by means of a further operation such as by lacquering, laminating, imprinting, etc.
The second capacitor plate is applied in one operation with the shielding, e.g., by means of electroplating, sputtering or lacquering.
The capacity of the feed-through capacitor constructed in accordance with this invention may be calculated as follows:
Two-plate capacitor:
C=∈xc2x7A/s;
∈=∈0xc2x7∈r, ∈0=8.85xc2x710xe2x88x9212 F/m
Cylinder capacitor:
C=2xcfx80xc2x7∈xc2x71/ln(r2/r1)
The impedance can be calculated as follows:
Xc=1/(2xcfx80xc2x7fxc2x7C)
The lower the frequency to be filtered, the greater the required capacity of the feed-through capacitor. A great capacity of the feed-through capacitor is achieved by means of large surfaces A and a small distance s of the capacitor plates.
The following advantages are achieved, with the following disadvantages eliminated:
elimination of the discrete feed-through capacitors and of all the production steps for their assembly;
the feed-through capacitors are manufactured without any additional operation, e.g., in but one cost-saving cycle of operation.
In the following, special examples of embodiments of this invention and their modes of operation will be described:
Two-plate capacitor:
C=∈xc2x7A/s;
∈r=4.2; A=2.5 mmxc2x77.5 mm; s=10 xcexcm (lacquered)
C=70 pF
Xc=1/(2xcfx80xc2x71.8 GHzxc2x770 pF)=1.3 xcexa9
Xc=1/(2xcfx80xc2x726 MHzxc2x770 pF)=87 xcexa9
∈r=4.2; A=2.5 mmxc2x77.5 mm;
s=0.3 mm (plastic, injection-moulded)
C=2.3 pF
Xc=1/(2xcfx80xc2x71.8 GHzxc2x72.3 pF)=38xcexa9
Cylinder capacitor:
C=2xcfx80xc2x7∈xc2x71/1n(r2/r1); 1=3 mm
∈r=4.2; r1=0.5 mm; r2=0.501 mm
C=350 pF
Xc=1/(2xcfx80xc2x71.8 GHzxc2x7350 pF)=250 mxcexa9
Xc=1/(2xcfx80xc2x71.8 GHzxc2x7350 pF)=17.5 xcexa9
The following variants of this invention are conceivable:
a combination of this invention with technologies already applied for representing conductor structures on plastic housings;
a special moulding of the capacitor plates for the manufacture of filter structures (e.g., T or pi filters) by using, e.g., the strip line technology.